The goal is to discover the molecular basis for the binding of merocyanine 540 to leukemia cells but not to normal blood cells. MC540 has been found to display a greater affinity for fluid phase bilayers than gel phase bilayers. Under the proper conditions, this difference can be converted to an all or nothing binding phenomenon: in the presence of 1 to 2% serum, leukemia cells stain but normal blood cells do not. Erythroleukemia cells and normal immature erythrocytes stain with a patchy distribution suggesting the existence of fluid phase state domains. When these cells differentiate, the domains are collected at the membrane location where enucleation takes place and are eliminated from the cell on the membrane which surrounds the extruding nucleus. It is suggested that these membrane domains serve as collection sites for obsolete membrane proteins which are thus marked for elimination from the cell. Work is currently in progress to determine whether such domains exist on other blood cells, and if they do, if their function is similar in these cell types. Finally, the potential use of MC540 in clinical medicine is being further investigated.